Infectious agents have infected prokaryotes and eukaryotes throughout evolution. Indeed, there is co-evolution among organisms and their infectious agents, with development of protective responses in the hosts and adaptive countermeasures to them by the infectious agents. One recently identified system of viral restriction is the Apolipoprotein B editing complex (APOBEC or A) family of proteins. Human APOBEC3G was first identified as an anti-viral factor in HIV infection. The human genome encodes multiple A3 proteins, including hA3G and hA3F. hA3G and hA3F restrict infection by Vif-deficient human immunodeficiency virus 1 (HIV-1). A3 proteins are packaged into virions and inhibit retroviral replication in newly infected cells, in part by deaminating cytosines on negative strand DNA intermediates and through as of yet uncharacterized mechanisms. We recently provided the first in vivo demonstration of an antiviral function for A3 proteins. We showed that mouse mammary tumor virus (MMTV) replication was inhibited by endogenous mA3 in vivo, since mice with targeted deletion of this gene were more susceptible to infection than their wild type littermates. We also showed that hA3G was packaged into MMTV virions and inhibited infection of cultured cells. We propose here to use MMTV to further probe the function of mA3 and hA3 proteins. We will examine the mechanism by which A3 proteins are packaged into virions, restrict retrovirus infection in vitro and in vivo, the role that polymorphisms in the A3 genes plays in affecting virus restriction and whether A3 expression in mammary tissue restricts milk-borne transmission of virus. As a consequence of these studies, we will know what role/s A3 proteins play in infection by exogenous viruses. These studies will provide a basis for understanding how this family of intrinsic immune factors inhibits viral infection of the mouse and other species by exogenous viruses, including HIV-1 infection of humans.